Pneumatic support colunm for a chair

ABSTRACT

A vertically adjustable column for supporting a seat of a chair. The column includes an outer guide tube mounted to a support stand, an intermediate telescoping tube slidably positioned with the guide tube and an inner telescoping tube slidably positioned within the intermediate tube. An upper portion of the inner telescoping tube is connected to the seat. A gas spring includes a cylinder mounted within the inner tube, a piston rod extending outwardly from the cylinder and a control pin assembly mounted to an upper portion of the cylinder. The piston rod is connected to the guide tube and is extensible between a collapsed position in which the cylinder and inner tube are substantially within the intermediate tube and the intermediate tube is substantially within the guide tube, and a raised position in which a portion of the cylinder and inner tube extend outwardly from the intermediate tube and a portion of the intermediate tube extends outwardly from the outer guide tube.

RELATION TO OTHER APPLICATIONS

This application is a division of U.S. application Ser. No. 08/347,475,filed Dec. 15, 1994, now abandoned which was nationalized from PCTapplication PCT/US93/05731, filed Jun. 14, 1993, which was acontinuation-in-part of U.S. application Ser. No. 07/898,907 filed Jun.15, 1992, now abandoned the entire disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to office chairs, and moreparticularly, to a height-adjustable, tiltable office chair whichsupports the body of a user in ergonomically desirable positions forperforming various tasks.

Office chairs are typically configured to allow tilting of the seat andbackrest as a unit or tilting of the backrest relative to the seat. Inchairs having a backrest pivotally attached to a seat in a conventionalmanner, the movement of the backrest relative to the seat can createshear forces acting on the legs and back of the user. These shear forcestend to cause an uncomfortable pulling of the user's clothing. In anattempt to compensate for these shear forces, some office chairs includea backrest which pivots while the seat tilts, such as those disclosed inU.S. Pat. Nos. 2,859,801 (Moore) and 4,429,917 (Diffrient). To provide achair which naturally conforms to the pivotal movement of both the legsand trunk of a user between tilt positions, it is desirable to provide achair having a seat and backrest which pivot generally about the axis ofthe hip joints of the user.

To further ensure comfortable tilting between tilt positions and enhancethe comfort of a user while in a given tilt position, it is desirable toprovide a chair having a tilt mechanism with an effective pivot pointabout the ankles of the user. Such an ankle tilt feature decreases theeffort required to tilt the chair, reduces the pressure of a forwardedge of the seat acting on the underside of a user's leg, and allows thefeet of the user to remain flat on a floor.

Although some tilting chairs have incorporated such an ankle tiltfeature, none have comprehensively addressed the overall body postureand relative positioning of body parts for ensuring comfort andminimizing fatigue regardless how intensely a user works. In most officeenvironments, a worker performs several tasks such as writing at a desk,dictating, using the telephone, or typing at a video display terminal(VDT). Not only do such tasks vary in the inherent intensity of the workbeing performed, but an individual may also desire to increase ordecrease the work intensity of a given task. As a result, the optimalposition of the body for ensuring comfort and minimizing fatigue alsovaries. Thus, it is desirable to provide a chair which automaticallysupports the body parts of a user in ergonomically optimal positions forperforming intensive, moderate, or relaxing modes of work. It is alsodesirable to provide adjustable armrests for positioning the arms of auser in optimal locations for various tasks regardless of the size andshape of a user.

A related disadvantage of conventional office chairs is theconfiguration and material of the seat and/or backrest. Such seatstypically include single or multi-density foam padding with a coveringsuch as cloth, leather or the like. This type of seating provides adeformable cushion which conforms to the user's buttocks. However, adeformable cushion does not provide a self-adjusting support whichvaries according to the position of the user and the tilt position ofthe seat. Such seating also tends to provide insufficient aeration sinceit acts as another layer of clothing. In chairs incorporating flexiblemembranes, the membranes are typically attached directly to the frame ofa seat. Often the membrane is attached to the frame by wrapping edgeportions of the membrane around spaced apart rods which define theframe. The membrane of such a seat is difficult to repair and/or replacesince the chair would typically have to be disassembled to allow suchmaintenance. In addition, the structural requirements of such anattachment limits the shape and size of the frame and the membrane.

Typically, the seats of office chairs are supported by a single stagetelescoping column which provides for vertical adjustment of the seat.These columns include a gas spring mounted in a telescoping tube whichis slidable within a base tube. In accordance with guidelines set by theAmerican National Standards Institute (A.N.S.I.) and Business andInstitutional Furniture Manufacturer's Association (B.I.F.M.A.),conventional office chairs in the United States are typically adjustablefrom a seat height of about 16.0 inches from a floor to about 20.5inches from a floor. Nevertheless, it is desirable to exceed this rangeof height adjustment to account for very small or large users and toaccommodate the international population in general.

Typically, it is difficult to exceed this range of height adjustmentwith seats which tilt about the knees or ankles of a user. To offset themoments acting on single stage support columns, pneumatic manufacturerstypically set a minimum overlapping distance of 2.95 inches (75 mm)between the tubes. Because such "ankle tilt" and "knee tilt" chairs haverelatively large tilt housings, it is difficult to provide a lowerminimum and higher maximum seat height while maintaining the requiredoverlapping distance between the tubes. These types of tilting chairsalso impart a greater moment on the tubes since the pivot axis is offsetfrom the support column. It is therefore desirable to provide avertically adjustable support column having a greater overlappingdistance to permit a greater stroke which decreases the minimum heightand increases the maximum height of a chair seat.

SUMMARY OF THE INVENTION

Briefly stated, the invention is directed to an office chair having aseat, a back and a pair of armrests which support the body of a user inergonomically desirable positions for performing various tasks.

In one aspect of the invention, a linkage assembly is adapted to allowthe seat and back to tilt downwardly and rearwardly and to allow pivotalmovement of the seat about a pivot axis in substantial alignment withthe hip joints of a user to inhibit shear forces from pulling theclothing on the body of a user.

In another aspect of the invention, the linkage assembly is adapted toallow the seat and back to tilt downwardly and rearwardly such that theseat pivots about an effective pivot point at substantially the ankle ofa user having feet resting on a floor.

In yet another aspect of the invention, a vertically adjustable columnincludes an outer guide tube, an intermediate telescoping tube slidablypositioned within the outer guide tube, and an inner telescoping tubeslidably positioned within the intermediate tube. The outer guide tubeis mounted to a support stand and the inner tube has an upper portionthereof connected to a seat support member. A gas spring is provided andincludes a cylinder mounted within the inner tube, a piston rodextending outwardly from the cylinder and having an end connected to abottom portion of the outer guide member, and a control pin assemblymounted to an upper portion of the cylinder for operable engagement withan actuator member. The piston rod is extensible between a collapsedposition in which the cylinder and inner tube are substantially withinthe intermediate tube and the intermediate tube is substantially withinthe outer guide tube, and a raised position in which a portion of thecylinder and inner tube extend outwardly from the intermediate tube anda portion of the intermediate tube extends outwardly from the outerguide tube.

In another aspect of the invention, the seat and back of the chairinclude a frame member having an central opening therethrough and areceptacle formed around the perimeter thereof. A membrane of elasticmaterial covers the central opening and is adapted to be attached to acarrier member which fits inside the receptacle.

In yet another aspect of the invention, a pair of armrests are adaptedto be mounted to the chair to allow pivotal movement thereof in a planesubstantially perpendicular to the general plane of the back toaccommodate the angle at which the forearms of a user are positioned.

In a preferred embodiment of the invention, the linkage assemblyincludes a pair of first links having an end pivotally attached to thelateral portions of the seat to define a pivot axis in substantialalignment with the hip joints of a user. A pair of second links have afirst section rigidly connected to the first links and the back and asecond section angled downwardly from the first section. The secondsection terminates in an end pivotally mounted to a forward portion ofthe base member. A restraining link has one end pivotally attached to arear portion of the seat and another end pivotally attached to the basemember to limit tilting of the seat. The first links, second links, andrestraining links are configured to create an effective pivot point ofthe seat at substantially the ankles of a user having feet resting on afloor. Thus, the seat pivots about the hip axis of a user such that theangle therebetween increases as the back and seat tilt rearwardly.

The chair preferably includes a bowed section to support the lumbarregion of a back of a user. The greater rearward tilting of the backrelative to the seat automatically lowers the bowed section toaccommodate the changing position of the lumbar region of the user.

Preferably, the chair also includes a height adjustable, flexible strapmember positioned horizontally across the back at approximately thelumbar region of the back of a user. The strap member is also laterallyadjustable to provide a desired tension for supporting the back of theuser. The armrests are preferably adapted to tilt with the back suchthat the angle therebetween remains constant during tilting. Preferably,the armrests are height adjustable relative to the back frame by a pawland rack mechanism.

In addition, the membrane of elastic material is preferablypre-stretched prior to insertion of the carrier member in the receptacleto accommodate for the contour of the body of a user. The carrier memberis preferably of one-piece construction and is adapted to be press-fitin the receptacle, and the edge portion of the membrane is preferablymolded with the carrier member. The carrier member can also beconfigured to snap-fit into the frame member receptacle, and the edgeportion of the membrane can be welded to the carrier member.

The embodiments of the present invention provide significant advantagesover other tiltable office chairs. For example, in the most preferredembodiment, the seat and back naturally conform to the body of a userduring tilting of the chair to thereby reduce shear forces acting on thethighs and trunk of the user and minimize pressure acting on theunderside of the user's thighs at the knees. This chair alsoautomatically supports the body of the user in ergonomically desirablepositions for performing tasks of varying intensity. The user's weightis distributed evenly and shifts of the center of gravity arecompensated for to allow the user to maintain equilibrium and goodbalance. Thus, the user's body is positioned to reduce fatigue, allowproper alignment for performing various tasks, achieve maximum alertnessor relaxation, and ensure overall comfort. Moreover, the range ofvertical adjustment of the chair is increased to allow a lower minimumheight and higher maximum height than conventional office chairs.

The present invention, together with further objects and advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a preferred embodiment of a chair havinga backrest, a seat, and a pair of armrests.

FIG. 2 is a front view of the chair.

FIG. 3 is a right side view of the chair.

FIG. 4 is a left side view of the chair.

FIG. 5 is a rear view of the chair.

FIG. 6 is a top view of the chair.

FIG. 7 is a bottom view of the chair.

FIG. 8 is a side view of the chair showing the seat and backrest in aforward tilt position.

FIG. 9 is a side view of the chair showing the seat and backrest in areclined tilt position.

FIG. 10 is a side view of the chair showing the seat and backrest inpreferred forward, middle, and reclined tilt positions.

FIG. 11A is a fragmentary side view of the chair in a raised positionshowing a preferred embodiment of a column assembly in cross-section.

FIG. 11B is a fragmentary side view of the chair in a lowered positionshowing the column assembly in FIG. 11A in a collapsed position.

FIG. 12A is a fragmentary side view of the chair in a raised positionshowing an alternative embodiment of the column assembly incross-section.

FIG. 12B is a fragmentary side view of the chair in a lowered positionshowing the column assembly in FIG. 12A in a collapsed position.

FIG. 13 is a cross-sectional view of yet another embodiment of thecolumn assembly shown in FIGS. 11A and 11B.

FIG. 14 is a top view of the chair showing the armrests in variouspivoted positions indicated by broken lines.

FIG. 15 is a fragmentary perspective view of the chair showing anexploded view of an armrest assembly, a backrest frame and a linkageassembly.

FIGS. 16 and 17 are exploded top views of an alternative embodiment ofan armrest assembly.

FIG. 18 is a cross-sectional view of the armrest assembly shown in FIGS.16 and 17.

FIG. 19 is a fragmentary rear view of an alternative embodiment of thebackrest and a brace member.

FIG. 20 is a cross-sectional view of the backrest and brace member shownin FIG. 19.

FIG. 21 is a front view of a tilt control mechanism with variousportions removed for clarity.

FIG. 22 is a cross-sectional view of the tilt control mechanism takenalong the line 22--22 in FIG. 21.

FIG. 23 is a top view of the tilt control mechanism shown in FIGS. 21and 22 with various portions removed for clarity.

FIGS. 24 and 25 are side views of the tilt control mechanism showing arearward tilt limiter mechanism.

FIGS. 26 and 27 are side views of the tilt control mechanism showing aforward tilt limiter mechanism.

FIGS. 28 and 29 are cross-sectional views of mechanisms for actuatingthe tilt control mechanisms.

FIG. 30 is an exploded perspective view of the seat.

FIG. 31 is a perspective view of the seat shown in FIG. 30.

FIG. 32 is a cross-sectional view of the seat taken along the line32--32 in FIG. 31.

FIG. 33 is a cross-sectional view of the seat shown in FIG. 31.

FIG. 34 is a cross-sectional view of the seat taken along the line34--34 in FIG. 32.

FIG. 35 is a cross-sectional view of the seat shown in FIG. 31.

FIG. 36 is an exploded fragmentary view of a seat membrane.

FIGS. 37 and 38 are cross-sectional views of the membrane taken alongthe lines 37--37 and 38--38 in FIG. 36.

FIG. 39 is a top view of upper and lower loom members and clamp membersclamped to the seat membrane.

FIG. 40 is a cross-sectional view of the loom members, clamp members,and membrane taken along the line 40--40 in FIG. 39.

FIG. 41 is a top view of a lower mold member.

FIG. 42 is a top view of the loom member and membrane loaded in thelower mold member.

FIG. 43 is a cross-sectional view of the loom member, membrane and moldmembers taken along the line 43--43 in FIG. 42 and showing the moldmembers prior to closure.

FIG. 44 is a cross-sectional view of the mold members in FIG. 43 shownclosed against the loom member and membrane.

FIG. 45 is a cross-sectional view of the loom member, membrane and moldmembers taken along the line 45--45 in FIG. 42 and showing the moldmembers prior to closure.

FIG. 46 is a cross-sectional view of the mold members in FIG. 45 shownclosed against the loom member and membrane.

FIG. 47 is an exploded cross-sectional view of the closed mold membersin FIGS. 44 and 46 showing a cavity and the membrane.

FIG. 48 is a cross-sectional view of the mold members and membrane takenalong the line 48--48 in FIG. 47.

FIG. 49 is a perspective view of an alternative embodiment of the seat.

FIG. 50 is an exploded, cross-sectional view of the seat shown in FIG.49.

FIG. 51 is a perspective view of another alternative embodiment of theseat.

FIG. 52 is a cross-sectional view of the seat shown in FIG. 51.

FIG. 53 is a perspective view of another alternative embodiment of theseat.

FIGS. 54-56 are cross-sectional views of the seat shown in FIG. 53 takenalong the lines 54--54, 55--55, and 56--56 in FIG. 26.

FIG. 57A is an enlarged, partial side view of the column assembly ofFIG. 11A.

FIG. 57B is an enlarged, partial side view of the column assembly ofFIG. 11B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, FIGS. 1-7 show a preferred embodiment of achair 30 in a middle tilt position. The chair 30 includes a seat 32, abackrest 34, and a pair of armrest assemblies 36. The seat 32 andbackrest 34 are connected to a tilt control housing 38 by a linkageassembly 40. The tilt control housing 38 is mounted on a verticallyadjustable, dual stage support column 42 which is secured to the centerof a pedestal 44. The pedestal 44 is movably supported on a floor 46 bya plurality of casters 48 or the like.

In a preferred embodiment of the invention, the linkage assembly 40includes a pair of first links 50 pivotally attached to upwardlyextending side portions 52 of a seat frame 33 at pivot points 54 todefine a pivot axis at substantially the hip joints of a user. A pair ofsecond links 56 each have a substantially straight first section 58 towhich the first links 50 are fixedly attached and a second section 60angled downwardly from the first section 58. An upwardly extending rearend portion 62 of each first section 58 is connected to a frame 64 ofthe backrest 34, and an inwardly extending front end portion 66 of eachsecond section 60 is pivotally mounted to a forward portion of the tiltcontrol housing 38. Thus, the rigidly connected first links 50 andsecond links 56 act as two bars of a four bar linkage which creates aneffective pivot point 68 at substantially the ankles of a user havingfeet resting on the floor 46.

As best shown in FIGS. 8-10, the seat 32 and backrest 34 both pivotabout the hip pivot points 54 while simultaneously tilting rearwardly.To limit tilting of the seat 32, linkage assembly 40 includes a pair ofrestraining links 70 which form a four bar linkage in conjunction withthe first links 50 and second links 56. The restraining links 70 haveone end 72 pivotally attached to a front portion of the tilt controlhousing 38 rearwardly and below the attachment of the end portions 66 ofthe links 56 to the housing 38. Another end 74 of the restraining links70 is pivotally attached to a corresponding clevis 76 extendingdownwardly from a rear edge of the seat 32. Thus, the seat 32 pivotsabout hip pivot point 54 since it is pivotally mounted to the firstlinks 50, and the backrest 34 pivots about the same pivot point 54 sincethe second links 56 are fixedly connected to the first links 50. Inaddition, the seat 32 and backrest 34 simultaneously pivot about pivotpoint 66, and the restraining links 70 cause the seat 32 to pivot aboutthe effective pivot axis 68 at the ankles of a user.

One advantage of the foregoing chair configuration is the minimizing ofshear forces acting on the clothing worn on the legs and torso of theuser as the user tilts between various positions. Since the legs andtorso of a user naturally pivot about the hip joints, and both the seat32 and backrest 34 pivot about the same pivot axis 54, the clothing ofthe user will not pull when changing tilt positions. Although suchtilting of the seat 32 and backrest 34 is described in conjunction witha seat which pivots about the ankles of a user, the seat 32 and backrest34 can be adapted to pivot about other axes. For example, the secondlinks 56 can be configured such that a front end portion is pivotallymounted to the tilt housing assembly 38 directly above the supportcolumn 42 to provide a conventional "knuckle tilt". The second links 56can also be rigidly attached to the first links 50 and the first links50 can be pivotally mounted to the tilt control housing 38 at a desiredlocation.

The comfort of a user while tilting between various tilt positions isalso enhanced by the ankle tilt feature of the invention. Since the seat32 tilts rearwardly about the ankles of a user, the user can tiltrearwardly with little effort without lifting the feet off the floor.The configuration of the linkage assembly 40 also allows tilting of theseat 32 such that a forward edge portion 78 moves rearwardly withoutrising a substantial amount to minimize the pressure on the underside ofa user's thighs at the knees.

Another advantage of the invention is the positioning of the body of theuser in ergonomically desirable postures regardless of the task beingperformed or the intensity at which a user works. To this end, the seat32 and backrest 34 are tiltable between at least a forward, middle, andreclined tilt position corresponding to an intense, normal, and relaxedmode of work.

To increase attentiveness and minimize fatigue in a work intensive mode,it is desirable for the chair 30 to be tilted forwardly as shown inFIGS. 8 and 10. In this position, the seat 32 is tilted forwardly at anangle of approximately 6° from the floor 46 and the angle between theseat 32 and backrest 34 is approximately 95°. This tilt positionsupports the body of a user in a slightly leaning forward posture inwhich the user's feet are flat on the floor, the angle between the trunkand thighs is greater than 90°, the spinal centerline is approximatelyperpendicular to the floor 46, and the head of the user is eitherperpendicular to the floor or slightly bent downward and forward. Theforward tilt position tends to elevate blood pressure which enhancesreticular formation of the eyes and causes a state of alertnessdesirable for performing work-intensive tasks. The open angle betweenthe thighs and trunk of a user also enhances breathing to furtherenhance alertness.

To ensure comfort and minimize fatigue while maintaining the body in anattentive posture for normal modes of work, the chair 30 is tiltable toa middle position as illustrated in FIGS. 1-7 and shown in broken linesin FIG. 10. In this position, the seat 32 is substantially horizontaland the angle between the backrest and seat is approximately 104°. Thus,the body of user is supported in a slightly leaned back position inwhich the user's feet are flat on the floor, the angle between the trunkand thighs is increased, and the head of the user is perpendicular tothe floor.

In a reclined tilt position (FIGS. 9 and 10), the seat 32 is tiltedrearwardly at an angle approximately 11° from the floor 46 and the anglebetween the backrest and seat is approximately 108°. The body of a useris supported in a leaned back position in which the feet are flat on thefloor and further away from the chair 30 to open up the angle betweenthe calf and thighs of the user. The angle between the trunk and thighsof a user is increased and the head of the user remains perpendicular tothe floor. The trunk of the user is also angled back to the point wherea substantial weight shift occurs away from the buttocks to the spine,thereby relieving pressure and compression on the spinal discs.

To obtain the foregoing positions of the seat 32 relative to thebackrest 34, the first links 50, second links 56, and restraining links70 are configured to allow the angle between the seat 32 and backrest 34to increase as the seat and backrest are tilted rearwardly (FIGS. 8-10).Preferably, the angle between the backrest 34 and the floor 46 increasesat a greater rate than the angle between the seat 32 and the floor. Toprovide a desired resistance to rearward tilting of the seat 32 andbackrest 34 about ankle pivot axis 68, and further to limit the tiltingof the first links 50 and backrest 34 about hip pivot points 54, a tiltcontrol mechanism such as a torsion or compression spring is positionedin the tilt control housing 38. The second links 56 are biased forwardlyand upwardly by the tilt control mechanism which is described in moredetail below. In addition, an adjustable rearward tilt limiter mechanismis provided to vary the maximum rearward tilting of the chair 30 and aforward tilt limiter mechanism is provided to prevent forward tilting ofthe seat 32 past the generally horizontal middle position shown in FIGS.1-7. The rearward and forward tilt limiter mechanisms will be describedin more detail below. A tilt lock mechanism can also be provided to lockthe chair 30 in the forward, middle, and reclined tilt positions. Thiscan be accomplished by locking the second links 56 and backrest 34 inthe desired tilt position which also prevents movement of the seat 32.An example of this type of tilt mechanism is disclosed in U.S. Pat. Nos.4,555,085 (Bauer et al.) and 4,099,775 (Mizelle).

The chair 30 is also height adjustable to position the body of a varioussize users in ergonomically desirable positions relative to a floorand/or worksurface. The dual stage, vertically adjustable support column42 can be incorporated in any type of chair and is not limited to thechair 30 described herein.

In FIGS. 11A and 11B, the support column 42 includes an outer guide tube110 mounted to the pedestal 44 such that a bottom wall 112 thereof isspaced apart from the floor 46. An intermediate telescoping tube 114 isslidably positioned within the outer guide tube 110. The intermediatetube preferably 114 has an interior shoulder 116 and an exteriorshoulder 118 at the middle of the tube to define a lower section 120having a larger inner and outer diameter than an upper section 122. Thelower section 120 of the intermediate tube 114 slidably bears againstthe outer tube 110, and when locked in a desired position, theoverlapping area of the outer tube 110 and intermediate tube lowersection 120 offsets any moments acting on the tubes to support a usersitting on the chair 30. To limit the upward travel of the intermediatetube 114, a retaining collar 124 is mounted to the top of the outer tube110 and slidably receives the upper section 122 of the intermediate tube114. In a raised position, the exterior shoulder 118 of the intermediatetube 114 bears against the collar 124 of the outer tube 110.

An inner telescoping tube 126 is slidably positioned within theintermediate tube 114 and has a top portion which is mounted to the tiltcontrol housing 38. The inner tube 126 slidably bears against the uppersection 122 of the intermediate tube 114, and when locked in a desiredposition, the overlapping area of the inner tube 126 and intermediatetube upper section 122 further offsets any moments acting on the tubesto support a user sitting on the chair 30. In addition, the momentacting on the tubes is minimized because an upper edge 128 of theintermediate tube 114 is closer to the tilt housing 38 than conventionalsupport columns, thus decreasing the moment arm acting on the tubes. Tolimit the upward travel of the inner tube 126, a retaining collar 130 ismounted to a bottom edge of the inner tube 126 and slidably bearsagainst the lower section 120 of the intermediate tube 114. Theretaining collar 130 also carries the intermediate tube 114 therewithwhen the inner tube 126 moves upwardly.

To adjust the vertical position of the chair, a conventional gas spring132 including a pneumatic cylinder 134 is mounted within the inner tube126. A piston rod 136 extends outwardly from the cylinder 134 in anaxial direction and has an end 138 connected to the bottom wall 112 ofthe outer guide tube 110. A control pin 140 extends upwardly from a topwall of the cylinder 134 for operable engagement with a conventionalactuator member (not shown). Preferably, the actuator member isactivated by a control knob on the end of a cable (not shown) which ishoused on the first link 50. The piston rod 136 is extensible between acollapsed position (FIG. 12) and a raised position (FIG. 11). In thecollapsed position, the cylinder 134 and inner tube 126 aresubstantially within the intermediate tube 114 and the intermediate tube114 is substantially within the outer tube 110. In the raised position,a portion of the cylinder 134 and inner tube 126 extends outwardly fromthe intermediate tube 114 and the upper section 122 of the intermediatetube 114 extends outwardly from the outer tube 110.

Thus, the intermediate tube 114 provides an additional overlappingsupport area and decreases the moment arm which would otherwise act onthe outer tube 110 to allow the tilt housing 38 and seat 32 to be raisedto a greater height. The additional height obtainable by the tilthousing 38 and seat 32 because of the intermediate tube 114 alsodecreases the required height of the outer tube 110. As a result, thechair 30 can be lowered to a lower as well as a higher position thanconventional chairs. Preferably, the distance between the floor 46 andthe bottom wall 112 of the outer tube 110 is approximately 1/2 inch andthe height of the outer tube 110 is approximately 81/2 inches to allowthe tilt housing 38 to be lowered to a height of approximately 9 inchesfrom the floor. Moreover, the stroke of the piston rod 136 is preferablyabout 7 inches to allow the tilt housing 38 to be raised to a height ofapproximately 16 inches from the floor.

FIGS. 12A-13 illustrate alternative embodiments of the support column42. Since these embodiments are similar to the previously describedembodiment, similar parts appearing in FIGS. 12A-13 are represented bythe same reference numerals. Referring now to FIGS. 12A and 12B, theintermediate tube 114, preferably made of steel, is substantiallycylindrical and radially spaced from the outer tube 110 and inner tube126. To guide and support the intermediate tube 114 within the outertube 110, a bushing 124 extends radially inward from a top portion ofthe outer tube 110, and a lower bushing 111 extends radially outwardfrom a bottom portion of the intermediate tube 114. Thus, the bushing124 slidably bears against an exterior surface 113 of the intermediatetube 114 and the lower bushing 111 slidably bears against an interiorsurface 115 of the outer tube 110 when the intermediate tube 114 movesaxially within the outer tube 110. When a user sits on the chair, thedistance between the load-bearing bushings 111 and 124 defines a momentarm which acts to offset any moments acting on the intermediate tube114. To limit upward movement of the intermediate tube 114 within theouter tube 110, a first spacer 117, preferably in the form of an annularband, is positioned between the outer tube 110, intermediate tube 114,and bushings 111 and 124. As illustrated in FIG. 12B, the first spacer117 preferably fits loosely between the tubes 110 and 114 so that itremains seated on the intermediate tube lower bushing 111 as the bushingmoves downwardly with the intermediate tube 114.

The inner telescoping tube 126 is likewise radially spaced from theintermediate tube 114 and is preferably cylindrical and made of steel.To support and guide the inner tube 126 within the intermediate tube114, a bushing 130 extends radially outward from a bottom portion of theinner tube 126, and an upper bushing 119 extends radially inward from atop portion of the intermediate tube 114. To guide the inner tube 126within the intermediate tube 114, the inner tube bushing 130 slidablybears against an interior surface 121 of the intermediate tube 114 andthe intermediate tube upper bushing 119 slidably bears against anexterior surface 123 of the inner tube 126. Thus, the distance betweenthe load-bearing bushings 119 and 130 defines a moment arm which acts tooffset any moments acting on the inner tube 126. To limit the upwardtravel of the inner tube 126, a second spacer 125, preferably in theform of an annular band, is positioned between the inner tube 126,intermediate tube 114, and bushings 130 and 119. As shown in FIG. 12A,the maximum height of the chair is limited by the height of the firstand second annular spacers 117 and 125 which bear against the bushings111, 119, 124 and 130.

Therefore, the overlapping distance between the tubes 110, 114 and 126,or more particularly, the distance between the load-bearing bushings111, 119, 124 and 130 provides more lateral support than conventionalcolumns. As a result, the tilt housing 38 and seat 32 can be raised to agreater maximum height and lowered to a lower minimum height. Moreover,the embodiment of column 42 is less costly than conventional supportcolumns which typically have a single telescoping tube slidably bearingagainst a sleeve which is mounted within a top portion of the outertube. A relatively tight tolerance must be held between the sleeve andthe telescoping tube in these columns to prevent any angular movement orswaying of the tube. Because the present invention utilizes a pluralityof spaced apart, load bearing bushings positioned at the ends of thetubes, the tendency for the intermediate tube 114 and inner tube 126 tosway is reduced, thus obviating the need for such a tight tolerance.

Another aspect of the embodiment shown in FIGS. 12A and 12B is afrusto-conically shaped mounting member 127 attached to an upper portionof the outer tube 110. The outer surface of the mounting member 127 hasa relatively large taper and mates with a frusto-conically shaped cavity129 formed in the pedestal or support stand 44 which also has arelatively large taper. Preferably, the cavity 129 is formed in a centerportion 131 of the pedestal 44 and is defined by a hub 133 extendingdownwardly therefrom. The tapered mounting member 127 fits within anupper portion of the cavity 129, and a lower portion of the outer tube110 bears against an inner wall 135 of the cavity 129 at a lower portionthereof, which provides additional lateral support for the column 42.Typically, the outer tubes of conventional support columns have acylindrical top portion extending out of a pedestal and a slightlytapered bottom portion mounted directly to a similarly shaped cavity inthe pedestal. If the bottom portion of the outer tube does not fitexactly within the cavity when assembled to the pedestal, the slighttapers may prevent the outer tube from dropping entirely within thecavity which raises the minimum height of the seat. Thus, the slightlytapered portion of conventional outer tubes typically require a tighttolerance in order to properly fit within a cavity in the pedestal. Suchtight tolerances are difficult to maintain and costly. The mountingmember 127 and cavity 129 obviates this problem by providing a largertaper which allows the outer tube 110 to drop entirely within the cavity129. In addition, the outer tube 110 does not require as tight atolerance since the upper portion is not mounted directly to thepedestal 44.

Another difficulty with conventional support columns is that the topportion of the outer tubes typically cannot be tapered since the stressacting on the bearing sleeves, which are typically mounted within thetop portion of the outer tubes, tends to deform the sleeves. Asdiscussed above, such deformation is unacceptable since a tighttolerance must be held between the sleeves and the outer tubes. As aresult, the cylindrical top portions of the outer tubes often extend outof the pedestal which further reduces the minimum height of the seat.Because the column 42 does not require such tight tolerances between itscomponents, and since a plurality of load-bearing members are movablyspaced from the top portion of the outer tube 110, the frusto-conicalmounting member 127 can be mounted to the top portion of the outer tube110 without fear of deformation which could bind the tubes. As a result,the top portion of the outer tube 110 can be positioned within thecavity 129 to further reduce the minimum height of the seat 32.

FIG. 13 illustrates yet another embodiment of the support column 42which includes a spring 142 positioned within a bottom portion of theouter guide tube 110. The intermediate tube 114 does not have aninterior shoulder for engagement with a retaining bushing to raise theintermediate tube with the inner tube 126. Rather, the spring 142engages a bottom edge 144 of the intermediate tube 114 to bias theintermediate tube upwardly.

Another aspect of the invention is the height adjustable, pivotalarmrests 36. As best shown in FIG. 14, the armrests 36 are pivotal aboutaxes adjacent side edge portions of the back. The axes are positionedfor approximate alignment with the elbows of a user when the user'sforearms are resting on the armrests 36 to accommodate the angle atwhich the forearms are positioned. The armrests 36 are mounted to thebackrest frame 64 to ensure proper alignment with the forearms of a userin any tilt position.

Referring now to FIG. 15, the armrest 36 includes a base plate 150 onwhich a pad is mounted and a support arm 152 having a substantiallyhorizontal top plate 154. The base plate 150 has a threaded hole 156therein and the top plate 154 has a hole 158 therein for receiving athreaded fastener 160 which allows the base plate 150 to pivot relativeto the support arm 152. A detent member 162 extends upwardly from thetop plate 154 for operable engagement with an irregular surface on theunderside of the base plate 150 (not shown). The detent member 162 isbiased in a direction toward the irregular surface on the base plate 150by a spring or the like to lock the base plate 150 in a desiredposition. The armrests 36 are preferably pivotal approximately 20°inward and 10° outward from a vertical plane substantially normal to thegeneral plane of the backrest 34. Thus, the armrests 36 are pivotal to adesired angle to ensure sufficient contact with the forearms of a userto accommodate various size users and to ensure proper alignment withvarious work devices such as narrow keyboards or the like.

Again referring to FIG. 15, the support arm 152 includes a cavity 164defined by spaced apart side walls 166, an end wall 167 having a radius,and a top wall 168. A pawl 170 is positioned in the cavity 164 forpivotal movement about a pin 172 which extends between the side walls166 of the support arm 152. An actuation button 174 extends downwardlyfrom an upper end of the pawl 170 for pivoting a lower end portion 175of the pawl 170 into and out of engagement with a plurality of teeth 176extending outwardly from the side member 90 of backrest frame 64. Theend portion 175 of the pawl 170 is normally biased toward the teeth 176by a spring or the like. The upwardly extending rear end portion 62 ofthe first link 56 is rigidly mounted to the side member 90 of backrestframe 64 by conventional fasteners (not shown) and is preferablyconfigured as a sleeve member having a vertical slot 178 therein forslidably receiving the pawl 170. A guide member 180 is positionedbetween the sleeve member 62 and the teeth 176 of the backrest frameside members 90. The guide member 180 has a rounded surface 182 whichslidably bears against the sleeve member 62 and a slot 184 thereinsubstantially the same size and shape as the end portion 175 of the pawl170 for receiving the end portion 175. The guide member 180 also has aplurality of threaded holes 186 therein and the end wall 167 of thesupport arm 152 has threaded holes 188 therein for receivingconventional fasteners (not shown) which slidably hold the support arm152 flush against the sleeve member 62.

In operation, the actuation button 174 is depressed to pivot the pawl170 about pin 172 to disengage the end portion 186 from the teeth 176 onthe backrest frame side member 90. The support arm 152 is slidably movedto a desired height, and the actuation button 174 is released to engagethe end portion 186 of the pawl 170 with the teeth 176 and lock thesupport arm 152 in the new position.

FIGS. 16-18 illustrate an alternative embodiment of the armrests 36. Thetop plate 154 of the support arm 152 has an index bushing 190 extendingupwardly from a forward portion thereof and a pivot bushing 192extending upwardly from a rear portion thereof. The base plate 150 has ahole 194 in a rear portion thereof for receiving the pivot bushing 192and a curvilinear slot 196 therein which is positioned forwardly of thehole 194 for receiving the index bushing 190. The base plate 150 ispivotable about the pivot bushing 192 and the curvilinear slot 196 has aconstant radius about the pivot bushing 192. To provide intermittentstops for the base plate 150, an intermediate plate 198 is positionedbetween the base plate 150 and the top plate 154 of the support arm 152.The intermediate plate 198 is attached to an underside of the base plate150 and has a hole therein for receiving the pivot bushing 192. Theintermediate plate 198 also has an indexed slot 200 therein whichunderlies the slot 196 in the base plate 150 and receives the indexbushing 190. The indexed slot 200 has a centerline with the sameconstant radius as the slot 196. Preferably, the slot 200 is defined byan edge 201 approximating three circular portions 202, 204, and 206having a slightly larger diameter than the index bushing 190. Thecircular edge portions 202, 204 and 206 are configured to retain theindex bushing 190 yet allow it to pass from one circular portion to thenext upon the application of a lateral force on the intermediate plate198. Thus, the base plate 150 can be pivoted between three lockedpositions defined by the circular edge portions 202, 204 and 206 of theintermediate plate 198. Preferably, the index bushing 190 and circularedge portions 202, 204 and 206 are oriented to lock the base plate 150in a first position perpendicular to the general plane of the backrest34, a second position 20° inward from the first position, and a thirdposition 10° outward from the first position as shown in FIG. 14. Toprovide other angular positions of the armrests 36, the indexed slot 200can be provided with a desired number of circular edge portions atdesired angles relative to the pivot bushing 192. In addition, the baseplates 150 and cushions have a bowed portion 208 which provide acomfortable support for the user's forearms when the chair 30 is in anytilt position.

Other features of the chair 30 are provided which conform to the body ofa user between tilt positions and assist in supporting the body inergonomically desirable positions. For example, the seat 32 includes aself-adjusting elastic membrane 210 and the backrest 34 includes asimilar membrane 212 for comfortably supporting the user in any tiltposition. The membranes 210 and 212 and the manner in which they areattached to the seat and backrest frames 33 and 64 will be described inmore detail below.

To support the lumbar region of a user's back, the frame 64 of thebackrest 34 includes a bowed section 214 and the membrane 212 includes acorresponding bowed section 216. Since the angle between the backrest 34and the floor 46 increases at a greater rate than the angle between theseat 32 and the floor as the chair is tilted rearwardly, the bowedsections 214 and 216 of the backrest 34 automatically move downwardly,preferably a distance of about 1.5 inches between the forward tilt andthe reclined positions, to insure proper positioning of the lumbarsupport in any tilt position.

To further adjust the positioning of the lumbar support, an adjustablebrace member 218 is attached to side members 220 of the backrest frame64. The brace member 218 is positioned horizontally between the sidemembers 220 and behind the backrest membrane 212. FIG. 15 shows oneembodiment of a brace member 218 which is made of a flexible materialsuch as woven nylon or the like. The brace member 218 includes hookmembers 222 received by a vertical slot 224 formed in the side members220 of the backrest frame 64. To adjust the height of the brace member218, a conventional fastener such as VELCRO® or the like (not shown) isdisengaged to decrease the tension in the brace and allow movement ofthe hook members 222 within the slots 224. To reconnect the brace member218 in a desired location and/or laterally adjust the brace member toobtain a desired tension, the fastener is tightened a desired amount.

FIGS. 19 and 20 illustrate another embodiment of the brace member 218.In this embodiment, the backrest frame 64 is angled relative to themembrane 212 so that a rear inner edge 226 of the frame 64 is spacedapart from the membrane 212. The brace member 218 is preferably ovalshaped and made of a relatively soft yet semi-rigid material such asrubber or the like. As shown in FIG. 20, the brace member 218 has aninner surface 228 which bears against the backrest membrane 212. Thebrace member 218 is rotatably attached to the inner edge 226 of thebackrest frame 64 to allow adjustment of the angle of the brace member218. This angular adjustment stretches the membrane 212 and provides thedesired location and amount of support for the lumbar region of a user'sback. For example, the brace member 218 can be rotated from a standardposition shown in solid lines in FIG. 20 to a different position shownin broken lines in FIG. 20. Preferably, an upper edge portion 230 and alower edge portion 232 of the brace member 218 each have a radius whichprovides a comfortable surface area supporting the user's back when thebrace member 218 is rotated to an angle relative to the membrane 212.

The brace member 218 is rotatably attached to the frame 64 by a pair ofswivel connectors 234, 236 which are mounted to the ends of the bracemember 218. The structure of the connectors 234 and 236 is substantiallyidentical and will be described particularly only with reference to theconnector 234. The connector 234 has a plurality of vertically alignedhook members 238 extending perpendicularly outward from a plate 240 forgrasping the frame edge 226. A pivot rod 242 extends perpendicularlyinward from the plate 240 and is received by an insert 244 in a ball andsocket type arrangement. The insert 244 is mounted within a cavity 246in the brace member 218 and has a plurality of annular ribs 248 whichare received by corresponding annular grooves in the cavity to preventaxial displacement of the insert 244. The pivot rod 242 is preferablymounted within the insert 244 with sufficient frictional engagement torequire manipulation of the brace member 218 in order to pivot the bracemember.

Thus, an adjustable brace member 218 is provided which supports thelumbar region of a user's back a desired amount at a desired location.The connectors 234 and 236 can also be adapted for releasable attachmentto the frame edge 226 to allow vertical adjustment of the brace member218.

FIGS. 21-27 illustrate the tilt control mechanism of the presentinvention. As described above, the inwardly extending front end portions66 of links 56 are pivotally attached to a forward portion of the tiltcontrol housing 38. The ends 72 of the restraining links 70 arepivotally attached to the forward portion of the housing 38 rearwardly,and below the attachment of the end portions 66 of links 56. Preferably,the end portions 66 of links 56 are rigidly attached to a hexagonal axle250 which extends transversely through the housing 38 and is rotatablyattached to spaced apart side walls 251, 252 of the housing 38 via apair of bushings 254. Likewise, the ends 72 of the restraining links 70are rigidly attached to a transversely extending bar 256 which isrotatably attached to the side walls 251, 252 of the housing 38. Toprovide a restoring torque against the rearward tilting of the seat 32,an elastomeric torsion spring 258 is mounted to the hexagonal axle 250.The spring 258 is mounted for twisting movement about the axle 250 toresist rotation of the axle 250 when a user sits on the seat 32. Atorsion spring of this type is manufactured by the B.F. Goodrich Companyand is designated as TORSILASTIC® spring. As shown in FIGS. 21-23, abushing 260 having a hexagonal core is fixedly mounted to the hexagonalaxle 250, and a molded sleeve 262 of a rubber-like elastomeric materialis fixedly mounted to the bushing 260. An outer metal sleeve 264 isfixedly attached to the elastomeric sleeve 262, and an arm 266 extendsradially outward from the outer sleeve 264. To provide a restoringtorque against rotation of the axle 250, the arm 266 is adapted to befixed to the housing 38. Thus, the rotation of the axle 250 resultingfrom a user sitting on the seat 32 causes the elastomeric sleeve 262 totwist which exerts a restoring torque against the axle 250.

The initial restoring torque exerted by the spring 258 against therotation of axle 250 can be adjusted by changing the position of theouter sleeve arm 266. To provide easy adjustment, the outer sleeve arm266 has spaced apart side portions 268 defining a recess 270 in the endof the arm 266. The side portions 268 operably engage a transverselyoriented block member 272 which is threadably attached to a screw 274.The screw 274 is mounted to a bottom wall 276 of the housing 38 andextends upwardly through the recess 270 in the arm 266. The axis of thescrew 274 is positioned generally tangentially to the outer sleeve 264,and a bevel gear 278 is attached to a top portion 280 of the screw 274.A bevel gear 282 meshes with the bevel gear 278 and has an axis whichintersects the axis of the bevel gear 278. The bevel gear 282 isattached to the end of a shaft 284 which is rotatably mounted to theside wall 251 of housing 38. The shaft 284 extends horizontally outwardfrom the side wall 251 of the housing and has a handle 286 thereon toprovide easy access for a user. In operation, the shaft 284 and gear 282are rotated a desired amount to rotate the gear 278 which in turnrotates the screw 274. Rotation of the screw 274 causes the block member272 to move linearly along the axis of the screw 274, which moves theouter sleeve arm 266 of to the desired radial location. Preferably, thegear ratio of bevel gears 278 and 282 is such that a minimal amount ofeffort is required to move the arm 266. Thus, the outer sleeve arm 266is easily moved a desired amount to vary the initial restoring torque ofthe spring 258 and thereby control the rate at which the seat 32 andback 34 tilts rearwardly when a user sits on the seat 32.

An adjustable rearward tilt limiter mechanism 290 is also provided tovary the maximum rearward tilting of the seat 32 and backrest 34. Asbest shown in FIG. 24, a cam member 292 and gear 294 are mounted to arod 296 which is rotatably mounted to the side wall 252 of housing 38.The can member 292 preferably has a plurality of concave surfaces 298formed in an outer edge 300 thereof. An arm 302 is fixedly mounted tothe axle 250 and has a convex follower member 304 attached to an endthereof. The arm 302 extends rearwardly from the axle 250 such that thefollower member 304 is in operable engagement with one of the concavesurfaces 298 of the cam member 292 when a user sits on the seat 32. Asviewed in FIGS. 24 and 25, the maximum clockwise rotation of the axle250 and therefore the maximum rearward tilt position of the seat 32 andbackrest 34 is determined by the position of the cam member 292. Toadjust the position of the cam member 292, a pie-shaped member 306 isrotatably attached to the side wall 252 of the housing 38. The member306 has a plurality of teeth 308 on a circular edge portion thereofwhich mesh with the gear 294. A spring 310 is attached to the pie-shapedmember 306 and the side wall 252 of the housing 38 to bias rotation ofthe member 306 in a clockwise direction. A cable 312 is attached to themember 306 opposite the spring 310 and guided within a guide member 314which is attached to the side wall 252 of the housing 38. In operation,the cable 312 is moved axially a desired amount to rotate the pie-shapedmember 306, which in turn meshes with the gear 294 to rotate the cammember 292 to a desired position. When the chair is tilted rearwardly,one of the concave surfaces 298 will act as a stop for the followermember 304 to limit the rearward tilting of the seat 32 and chair 34. Asshown in dotted lines in FIG. 25, the cam member 292 and arm 302 can berotated to lock the seat 32 and backrest 34 in a forward tilt position.

In addition, a forward tilt limiter mechanism 313 is provided to preventforward tilting of the seat 32 past the generally horizontal middleposition shown in FIGS. 1-7. As best shown in FIGS. 26-27, a pivotmember 314 is mounted to a rod 316 which is rotatably mounted to theside wall 251 of housing 38. The pivot member 314 has forward tiltabutment surface 318 and a standard tilt abutment surface 320. An arm322 is fixedly mounted to the axle 250 and has a load bearing member 324attached to an end thereof. The arm 322 extends rearwardly from the axle250 such that the load bearing member 324 is operably engageable witheither the forward tilt abutment surface 318 or the standard tiltabutment surface 320. As viewed in FIGS. 26 and 27, the maximumclockwise rotation of the axle 250 and therefore the maximum forwardtilt position of the seat 32 and backrest 34 is determined by theposition of the pivot member 314. To actuate the pivot member 314between the standard and forward tilt positions, a cable 326 is attachedto the pivot member 314. The cable member 326 is guided within a guidemember 328 which is attached to the side wall 251 of the housing 38. Inaddition, a spring 330 is attached to the side wall 251 of the housing38 and to the pivot member 314 opposite the cable 326 to bias rotationof the pivot member 314 in a counterclockwise direction as viewed inFIGS. 26 and 27. In operation, the cable 326 is moved axially a desiredamount to rotate the pivot member 314 so that the load bearing member324 is operably engageable with the standard tilt abutment surface 320as shown in FIG. 26 or with the forward tilt abutment surface 318 asshown in FIG. 27. When the chair is unoccupied or when a user leansforward, the pivot member 314 will act as a stop for the load bearingmember 324 to limit the forward tilting of the seat 32 and chair 34.

Preferably, the cables 312 and 326 are adapted to be actuated from alocation near the seat frame 32 so that a user does not have to leanover to adjust the tilt adjustment mechanisms 290 and 313. As shown inFIG. 28, a handle 332 can be configured for pivotal attachment within acavity 334 formed in one of the second links 50, the seat frame 33, orother convenient location. The cable 312 or 326 can thus be actuated bymerely pivoting the handle 332 a desired amount. Alternatively, a guidemember 336 and slot 338 can be provided to allow slidable movement of ahandle 340 to actuate the cable 312 or 326 as shown in FIG. 29.

Another feature of the chair 30 which assists in comfortably supportinga user in ergonomically desirable positions is the configuration of theseat 32. As best shown in FIGS. 30-35, the frame 33 of seat 32 supportsthe elastic membrane 210 across a central opening 352. Preferably, theframe 33 is a single molded piece of glass-filled thermoplasticpolyester and the membrane 210 includes a plurality of interwoven fibersas discussed in more detail below. To provide a rim which conforms tothe body of a user, the side portions 52 and a rear portion 354 of frame33 curve upwardly. To minimize pressure on the underside of a user'sthighs near the knees, especially when the chair 30 is tiltedrearwardly, a front portion 356 of the frame 33 curves downwardly. Themembrane 210 has a similar downwardly curving portion 357 which overliesthe front portion 356 of the frame 33. To further reduce the pressure onthe legs of a user, a cushion 358 made of polyurethane foam or similarmaterial fits in a recess 360 formed in the front portion 356 of theframe.

Preferably, the entire periphery of the membrane 210 is attached to aone-piece carrier member 362 which is removably received by a continuouschannel 364 in the seat frame 33. The channel 364 is formed in a topsurface 366 of the seat frame 33 and extends around the entire perimeterof the frame 33. Although the carrier member 362 is securely held by theframe 33 within the channel 364, a strip 367 is provided on theunderside of the carrier member 362 and an interlocking strip 369 isprovided on a bottom surface of the channel 364 to further secure thecarrier member 362 to the frame 33. The interlocking strips 367 and 369can be hook and loop type fasteners such as VELCRO® and can beconfigured as separate tabs spaced around the periphery of the carriermember 362 and channel 364. The carrier member 362 is preferably made ofa pliable yet semi-rigid thermoplastic polyester material such aspolybutylene terephthalate (PTB), polystyrene or glass-filledpolypropylene. The membrane 210 is preferably in-molded with the carriermember 362 as described in more detail below. Thus, the carrier member362 is deformable yet has sufficient rigidity to maintain the desiredcontour of the membrane 210 when inserted in the channel 364. To thisend, the carrier member 362 is formed with the same contour as thechannel 364 including a downwardly extending front portion similar tothe curvature of the front portion 356 of the frame 33.

In addition, a top surface 368 of the carrier member 362 is configuredto follow the contour of the top surface 366 of the frame at anylocation around the perimeter. Thus, the top surface 368 of the carriermember 362 has a variable slope which generally corresponds with thedownwardly extending front portion 356, the upwardly extending sideportions 52, and the upwardly extending rear portion 354 of the frame33. A smooth transition from the carrier member 362 to the frame 33 istherefore provided, which is especially desirable in the front portionof the seat where the legs of a user rest. To provide a smoothtransition from the membrane 210 to a front portion 370 of the carriermember 362, the periphery of the membrane 210 is attached to an upperinner corner 372 of the carrier member 362 at generally the same angleas the top surface 368 of the carrier member 362. The remaining portionof the membrane 210 is shown attached to the carrier member 362 at adifferent angle than the corresponding top surface 368 of the carriermember 362. However, it will be understood that the method for attachingthe membrane 210 to the carrier member 362 allows any desired"entry-angle" between the periphery of the membrane 210 and the carriermember 362.

As shown in FIGS. 36-38, the membrane 210 is preferably made of aplurality of elastomeric monofilaments 374 interlaced with a pluralityof strands 376 of fibrous yarn typically used in textile upholsteryweaving. The elastomeric monofilaments 374 are extruded from a blockcopolymer of polyetramethylene terephthalate polyester andpolytetramethylene ether. Preferably, this material is Hytrel® which isproduced by the E.I. DuPont DeNemours Company and has a durometer of 55on the D-scale, or more specifically, Hytrels grade 5544 or 5556. Themonofilaments 374 are extruded by standard industry techniques which arewell known to those skilled in the art. During the extrusion process,the monofilaments 374 are annealed while under tension to orient thepolyester molecules in one direction while leaving the poly ethermolecules unaffected. This increases both the tensile strength and themodulus of elasticity of the monofilaments 374.

Preferably, the block copolymer is extruded into 2350 deniermonofilaments having the following properties:

    ______________________________________                                        EX-120 (Eytrel 5556) Lot X-2174 Properties Summary 5556                                        Load @   Load @                                                               5%       10%    Load @ Elongation                                  Diameter (mils)                                                                          Elongation                                                                             Elongation                                                                           Break  @ Break                               Set No.                                                                             Min.    Max.   (g)    (g)    (g)    (%)                                 ______________________________________                                         1    16.5    24.9   215    455    4903   117                                       17.2    26.2   225    477    4803   113                                       16.6    24.9   210    457    5330   129                                       16.0    24.2   227    480    4980   122                                       16.0    24.2   213    461    5058   122                                 10    15.9    24.1   239    481    4967   125                                       16.6    25.1   221    455    5067   122                                       16.5    25.2   200    428    4944   124                                       16.1    24.3   211    441    4921   124                                       16.0    24.3   220    450    5121   128                                 20    16.6    25.1   244    486    5389   127                                       16.6    25.4   248    489    4958   123                                       17.5    26.5   233    472    4958   116                                       16.5    25.0   229    465    4999   126                                       15.8    23.9   225    455    4429   102                                 37    15.8    24.0   235    489    4835   123                                       15.9    24.1   246    515    4890   127                                       16.3    24.4   234    513    5266   131                                       16.4    25.1   193    464    4930   122                                       16.4    24.8   234    513    5198   129                                 Average                                                                             16.36   24.79  225.10 472.30 4997.30                                                                              122.60                              Hi    17.50   26.50  248.00 515.00 5389.00                                                                              131.00                              Low   15.80   23.90  193.00 428.00 4429.00                                                                              102.00                              ______________________________________                                    

The elastomeric monofilaments 374 are the primary load-carrying membersof the membrane 210 and preferably run laterally in the warp directionbetween the side portions 52 of the seat 32 to comfortably support auser. The monofilaments 374 conform to the shape of a user's buttocksand also conform to the natural movement of the body when the chair 30is in any tilt position. Preferably, the monofilaments 374 areprestretched between 6% and 9% elongation to maintain the desiredcontour of the membrane 210 prior to imparting a load on the membrane210. In addition, the prestretching produces the optimum conformingcharacteristics of the monofilaments 374. A plurality of elastomericmonofilaments can also run longitudinally in the weft direction betweenthe rear portion 354 and the front portion 356 of the seat 32 to providefurther support which may add to the comfort of the seat 32. Ifelastomeric monofilaments are provided in both the lateral andlongitudinal directions of the seat 32, the monofilaments in the lateraldirection can be pretensioned a desired amount and the monofilaments inthe longitudinal direction can be pretensioned a different amount toproduce the desired pressure distribution of the seat 32.

To provide greater comfort to a user, the cross-sections of the elasticmonofilaments 374 preferably have a width to height ratio in the rangeof 1.5/1 to 2/1. This provides greater comfort because the increasedwidth of the monofilaments provides a greater surface area forsupporting a user which distributes the forces acting on the user. Thus,the user feels less pressure from the individual monofilaments 374 asopposed to round monofilaments which are more like concentrated pressurepoints. In addition, the greater width of the monofilaments 374 createsa more opaque appearance of the membrane 210 which is attractive and maylessen the perception that the user is sitting on a net rather than aconventional cushion. In addition, the cross-section of themonofilaments 374 are preferably elliptical as shown in FIGS. 37 and 38to provide a less abrasive support. The monofilaments can be configuredwith various other cross-sectional shapes which are less abrasive than aconventional round monofilament. To extrude the monofilaments 374 intothe desired elliptical shape, the dies through which the block ofcopolymer material is drawn can have a octogonal cross-section.Preferably, the elliptical monofilaments, 374 have a width ofapproximately 0.02479 inches and a height or thickness of approximately0.01636 inches. With these dimensions, the membrane 210 has about 24-26monofilaments per inch in the lateral direction.

Referring again to FIGS. 36-38, the fiber strands 376 run longitudinallyin the weft direction of the seat 32 and are preferably arranged ingroups of three. Each strand 376 preferably includes adjacentmultifilament bundles 376A and 376B of spun, textured, or twisted 1500denier Nylon or polyester yarn. To provide additional support in thelongitudinal direction of the seat 32, an elastic monofilament 378 suchas spandex is incorporated into each strand 376 by spinning, air jettexturing or covering the monofilament 378. The monofilaments 378 arepreferably Lycra® monofilaments sold by the E.I. DuPont DeNemoursCompany, although other materials such as Hytrel® can be used to providethe desired support. The monofilaments 378 can be secured to the strands376 in any suitable manner such as wrapping the fibers of bundles 376Aor 376B around the monofilaments 378. In addition, a desired number ofmonofilaments 378 can be provided. The strands 376 are preferablyprestretched between 3% and 5% elongation in order to maintain thedesired contour of the membrane 210 with no load imparted on themembrane 210. In addition, the strands 376 are secondary load bearingmembers of the seat 32 and the prestretching produces the optimumconforming characteristics of the strands 376 when a user sits on themembrane 210. Preferably, the density of the strands 376 isapproximately 7-10 strands per inch.

As shown in FIG. 36, the strands 376 are interlaced with the elastomericmonofilaments 374 in an attractive, tightly woven pattern whichfacilitates aeration and provides a smooth seating surface. The strands376 are held in groups of three by pairs of the elastomericmonofilaments 374 which cross over between each group of strands. Forexample, monofilaments 374A and 374B are shown in FIG. 36 crossing overbetween a group 380 and a group 382 of strands 376. To maintain thespacing between each strand 376 in a group, the monofilaments 374 weavealternately above and below adjacent strands in the group. The pluralityof strands 376 provide a relatively large surface area of nonabrasivefabric which distributes the forces acting on a user to avoid a"grid-mark" type feel resulting from the concentration of pressure. Inaddition, the weave pattern provides sufficient aeration through theopenings between the monofilaments 374 and the strands 376 to allowevaporation of perspiration and facilitate air circulation to minimizeheat buildup. The longitudinal orientation and the grouping of thestrands 376 also provide an attractive seat with a longitudinal designand a relatively opaque appearance.

The method for forming the carrier member 362 and attaching the membrane210 thereto will now be described with reference to FIGS. 39-48. Asshown in FIGS. 39 and 40, a loom 384 having an upper member 386 and alower member 388 is provided to capture and hold the membrane 210 in astretched condition. The lower member 388 of loom 384 is initiallyplaced in a stretching machine (not shown). The membrane 210 with thepreviously described weave pattern is then placed over the lower loommember 388, and clamp members 390 of the stretching machine clamp theedges of the membrane 210 and stretch it a predetermined amount in boththe lateral and longitudinal directions. The upper loom member 386 isthen clamped against the lower loom member 388 to hold the membrane 210in the stretched condition. The clamp members 390 of the stretchingmachine release the membrane 210, and excess edge portions 392 of themembrane 210 outside the loom are trimmed a desired amount.

The loom 384 has a semi-rectangular shape with a central opening largerthan the area defined by the carrier member 374. The loom 384 also has adownwardly extending front portion 394 which is similar to the contourof the downwardly extending front portion 356 of the frame 33. The upperand lower loom members 386 and 388 each include a frusto-conicallyshaped cover member 396 made of plastic such as an epoxy, urethane, orother suitable soft material which is molded over a bent steel tube 398.The steel tubes 398 are provided to give the loom members 386 and 388structural support and the plastic members 396 are provided protect asteel molding tool from chipping or other damage. The upper loom member386 has a pair of ridges 400 extending downwardly therefrom and thelower loom member 388 has a pair of matching grooves 402 formed thereinto secure the membrane 210 therebetween. A plurality of clamp devices(not shown) are also provided to hold the loom members together andmaintain the membrane 210 in the stretched condition.

As best shown in FIGS. 41-46, a molding tool 404 includes an upper moldmember 406 and a lower mold member 408. The upper and lower mold members406 and 408 have corresponding recesses 410 and 412 which are configuredto receive the upper and lower loom members 386 and 388. The moldmembers 406 and 408 also have upper and lower recesses 414 and 416 whichform a cavity 418 when the mold members 406 and 408 are closed together.The cavity 418 has the desired shape and contour of the carrier member362 and a substantial portion of the cavity 418 is the same shape andcontour of the channel a14 in the seat frame 33.

After the clamp members 390 of the stretching machine are removed andthe excess edge portions of the membrane 210 are trimmed, the assemblyof the loom 384 and stretched membrane 210 is removed for placement inthe lower mold member 408. The lower loom member 388 is then placed inthe recess 412 in the lower mold member 406 such that the membrane 210is shaped over a curved inner male portion 420 of the lower mold member408 as shown in FIG. 43. Thus, a periphery 422 of the membrane 210 ispositioned at the desired angle over the recess 412 in the lower mold408 and the prestretched membrane 210 attains the desired contour priorto closing the upper mold 406 against the lower mold 408. The upper mold406 is then closed against the lower mold 408 without further stretchingthe membrane 210 or changing the position of the periphery 422 of themembrane 210 over the recess 412. The loom recess 410 in the upper mold406 receives the upper loom member 386, and a female inner curvedportion 424 of the upper mold 406 which is the same contour as the maleinner curved portion 420 of the lower male mold 408 bears against themembrane 210. A plastic resin is then injected into the cavity 418 tosecure the periphery 422 of the membrane 210. The upper and lower moldmembers 406 and 408 are then pulled apart, the carrier member 362 andmembrane 210 assembly are removed, excess edge portions outside thecarrier member 362 are trimmed, and the interlocking strip 367 isadhesively bonded to the underside of the carrier frame 362.

FIGS. 47 and 48 illustrate the upper and lower mold members 406 in aclosed or "shut-off" position during which the resin is injected intothe cavity 418. Because the membrane 210 has a thickness, the moldmembers 406 and 408 cannot be completely clamped against each other. Theweaving of the monofilaments 374 and strands 376 creates a variablethickness membrane 210, and the closest the mold members 406 and 408 canbe clamped together is determined by the thickest portions of themembrane 210. As shown in FIGS. 47 and 48, the thickest portions of themembrane 210 are in the regions where the elastomeric monofilaments 374cross-over between the groups of strands 376. When the mold members 406and 408 are closed against the monofilaments 374 in the cross-overregion, the strands 376 and the single monofilaments 374 which are notoverlapping have less thickness, thus creating a gap 426 between themold members 406 and 408. It is desirable to minimize the gap 426 toinhibit the resin from leaking out of the cavity 418. Because theoverall thickness of the overlapping elliptical monofilaments 374 isless than the thickness of overlapping round monofilaments, the moldmembers 406 and 408 can be clamped closer together to minimize the gap426 and decrease the amount of leakage. In addition, the reduction inoverall thickness tends creates less stress when the mold members 406and 408 are clamped against the membrane 210 which tends to cause lessdamage to the monofilaments 374 and strands 376.

FIGS. 49-56 illustrate alternative embodiments of the seat 32. Sincethese embodiments are similar to the previously described embodiment,similar parts appearing in FIGS. 49-56 are represented by the samereference numerals. In FIGS. 49-56, the seat frame 33 includes aplurality of spaced apart slots 430 formed therein which extend througha bottom wall 432 of the channel a14. The carrier member 362 has aplurality of spaced apart tabs 434 extending downwardly from therefrom.The tabs 434 have a hook portion 436 extending outwardly from a lowerend thereof and the slots 430 are of sufficient size to receive the tabs434. The carrier member 362 is secured to the seat frame 33 by insertingthe carrier member into the channel a14 and snapping the tabs 434 intothe slots 430 such that the hook portions 436 extend through the slots430 and engage an underside 438 of the frame 33. This embodiment forattaching the carrier member 374 to the frame 33 can be used with anytype of seating utilizing a carrier frame regardless of the method usedto attach the seating surface to the carrier member.

FIGS. 49-56 also illustrate alternative embodiments for attaching themembrane 210 to the carrier frame 362. For example, the periphery 422 ofthe membrane 210 can be molded with an insert 440 which is press fitinside a cavity 442 formed in a top surface of the carrier member 362(FIGS. 49-50). The periphery 422 of the membrane 210 can also bevibration or sonic welded to a bottom surface 444 of the carrier member362 as shown in FIGS. 51-56, and a plurality of teeth members 446 canextend downwardly from the bottom surface 444 to further secure themembrane 210 thereto as shown in FIG. 52.

Preferably, the backrest 34 is constructed with the same materials andin the same manner as the seat 32, although the desired amount ofprestretching of the elastomeric monofilaments and strands may vary toreflect the different support required for the back of a user.

Pre-assembly of the seat and backrest membranes to the carrier membersfacilitates maintenance since the membrane/carrier member assemblies canbe easily removed for repair and/or replacement. This configuration alsoprovides greater manufacturing flexibility. Since the carrier members donot have the structural requirements of a seat or backrest frame, adesired material can be used for a desired type of attachment method.For example, the material of the carrier member can be chosen based onwhether the membrane is to be insert molded therewith or welded thereto.A semi-rigid material can be used to facilitate the insert moldingprocess and pre-stretching of the membrane, and a material having a lowmelting point can be used to facilitate welding of the membrane thereto.

The in-molding process for attaching the membranes to the carrier framesalso provides significant advantages over other attachment methods. Forexample, the configuration of the mold cavities can be varied to providethe frames and carrier members with any type of contour whichfacilitates design flexibility. The top surfaces of the frames andcarrier members can have the same slope to provide a smooth transitionbetween the frames and the carrier members. The shape of the membranesand the angle at which the membranes are attached to the carrier memberscan also be easily adjusted. In addition, the in-molding process allowsthe frames to be thinner because a relatively small channel is all thatis required for attachment of the carrier members to the frame.

The membranes also provide a flexible support which conforms to thenatural movement of the body of a user when the chair is in any tiltposition. FIGS. 2-10 show the approximate position of the seat andbackrest membranes when a user is sitting thereon. The membranes tend tominimize fatigue because they are responsive to micro-postural changesof a user which stimulates muscles in the trunk and allows spinalmovement to hydrate the spinal discs.

In the forward tilt position (FIGS. 8 and 10), the seat membrane 210maintains a forward angle of the pelvis which insures a proper curvatureof the spine. The elasticity of the seat membrane 210 allows the thighsof a user to slant forward while keeping the pelvis on a horizontalplane, thereby giving the user the sensation of not sliding out of thechair and reducing shear forces acting on the underside of the user'sthighs. The seat membrane 210 also passively positions the lumbar regionof a user's back against the bowed section 86 of the backrest membrane82. In the middle and reclined tilt positions shown in FIGS. 2-7 and 9,the elasticity of the seat membrane 210 automatically causes largerbuttocks to wedge more deeply into the pocket between the seat 32 andbackrest 34 to insure correct positioning of the user's lumbar regionagainst the bowed section 86 of the backrest mat.

The porosity of the backrest and seat membranes allows air to flowthrough the membranes to aerate the skin of a user. Providing suchaeration decreases uncomfortable heat buildup which would otherwiseoccur when a user sits for an extended period of time on conventionalchair upholstery which acts like another layer of clothing.

Thus, the tilt motion of the chair 32 and the resiliency of the seat andbackrest membranes passively stabilizes the pelvic-lumbar process toreduce muscle activity heretofore associated with leg crossing andslumping postures. In addition, the membranes accommodate for angularvariations in the sacral plates of various user's spinal columns.

Thus, in accordance with the most preferred embodiment, a chair isprovided which naturally conforms to the body of user during tilting ofthe chair to reduce shear forces acting on the thighs and trunk of theuser and minimize pressure acting on the underside of the user's thighsat the knees. The chair automatically supports the body of the user inergonomically desirable positions for performing tasks of varyingintensity, and the range of vertical adjustment of the chair allows alower minimum height and higher maximum height than conventional officechairs.

Although the present invention has been described with reference topreferred embodiments, those skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. As such, it is intended that the foregoingdetailed description be regarded as illustrative rather than limiting.It is the appended claims, including all equivalents thereof, which areintended to define the scope of the invention.

We claim:
 1. A vertically adjustable column for supporting a seat of achair, the column comprising:a guide tube mounted to a support stand,said guide tube comprising a first retaining member; an intermediatetelescoping tube slidably positioned within the guide tube, saidintermediate telescoping tube comprising an exterior shoulder and aninterior shoulder at substantially the middle of the length of saidintermediate tube, said exterior shoulder engaging said first retainingmember when in a raised position so as to limit the upward travel of theintermediate tube; an inner telescoping tube slidably positioned withinthe intermediate tube and comprising an upper portion thereof connectedto a seat support member and a second retaining member engaging saidinterior shoulder of said intermediate tube when in a raised position soas to limit the upward travel of the inner telescopic tube.
 2. Thecolumn of claim 1 further comprising a gas spring comprising a cylindermounted within said inner telescopic tube and a piston rod extendingfrom said cylinder in an axial direction and comprising an end connectedto said guide tube.
 3. The column of claim 2 wherein the distancebetween a floor and a bottom of the guide tube is approximately one halfinch and the height of the outer guide tube is approximately 81/2 inchesto allow the seat support member to be lowered to a height ofapproximately 9 inches from the floor, and the stroke of the piston rodis approximately 7 inches to allow the seat support member to be raisedto a height of approximately 16 inches from the floor.
 4. The column ofclaim 1 wherein said interior shoulder is positioned below said exteriorshoulder.
 5. The column of claim 1 wherein said first retaining membercomprises a collar mounted to a top portion of said guide tube.
 6. Thecolumn of claim 1 wherein said collar has an inner diameter smaller thanthe inner diameter of the guide tube.
 7. The column of claim 1 whereinsaid second retaining member comprises a collar mounted to a bottomportion of said inner telescopic tube.
 8. The column of claim 1 whereinsaid first retaining member comprises a collar mounted to a top portionof said guide tube and said second retaining member comprises a collarmounted to a bottom portion of said inner telescopic tube.
 9. The columnof claim 1 wherein said exterior shoulder is at substantially the middleof the length of the intermediate tube.
 10. A vertically adjustablecolumn comprising:a support stand; a guide tube; an intermediatetelescoping tube slidably positioned within the guide tube; an innertelescoping tube slidably positioned within the intermediate tube andcomprising an upper portion thereof connected to a support member; afrusto-conically shaped mounting member attached to the guide tube andmating with a frusto-conically shaped cavity formed in the supportstand.
 11. The column of claim 10 further comprising a gas springcomprising a cylinder mounted within said inner telescopic tube and apiston rod extending from said cylinder in an axial direction andcomprising an end connected to said guide tube.
 12. The column of claim10 wherein said mounting member fits within an upper portion of thecavity.
 13. The column of claim 10 wherein said mounting member fitswithin an upper portion of the cavity and a lower portion of the guidetube bears against an inner wall of the cavity at a lower portionthereof to securely mount the column to the support stand.
 14. Thecolumn of claim 13 wherein the support stand comprises a plurality oflegs extending radially outward from a center portion thereof, saidcavity being formed in said center portion and being further defined bya hub extending downwardly from the juncture of said legs, therebyproviding lateral support for the bottom of the guide tube to allow saidguide tube to be mounted in a lower position relative to a floor. 15.The column of claim 10 wherein said mounting member is attached to anupper portion of the guide tube.
 16. The column of claim 10 wherein saidguide tube comprises a bushing attached to a top portion thereof, andsaid intermediate tube comprises a lower bushing mounted to a lowerportion thereof, said intermediate tube slidably bearing against theguide tube bushing, and said lower bushing slidably bearing against saidguide tube.
 17. The column of claim 16 further comprising a spacerdisposed between the guide tube and the intermediate tube and positionedbetween the guide tube bushing and the lower bushing of saidintermediate tube, said spacer engaging said guide tube bushing and saidlower bushing when said intermediate tube is in a raised position so asto limit the upward travel of said intermediate tube.
 18. The column ofclaim 10 wherein said intermediate tube comprises an upper bushingmounted to an upper portion thereof, and said inner tube comprises abushing mounted to a lower portion thereof, said inner tube slidablybearing against the upper bushing of said intermediate tube, and saidbushing of said inner tube slidably bearing against said intermediatetube.
 19. The column of claim 18 further comprising a spacer disposedbetween the intermediate tube and the inner tube and positioned betweenthe upper bushing of said intermediate tube and the bushing of saidinner tube, said spacer engaging said upper bushing of said intermediateguide tube and said bushing of said inner tube when said inner tube isin a raised position so as to limit the upward travel of said innertube.
 20. The column of claim 10 wherein said guide tube comprises abushing attached to a top portion thereof, said intermediate tubecomprises a lower bushing mounted to a lower portion thereof and anupper bushing mounted to an upper portion thereof, and said inner tubecomprises a bushing mounted to a lower portion thereof, saidintermediate tube slidably bearing against the guide tube bushing, saidlower bushing of said intermediate tube slidably bearing against saidguide tube, said inner tube slidably bearing against the upper bushingof said intermediate tube, and said bushing of said inner tube slidablybearing against said intermediate tube, and a first spacer disposedbetween the guide tube and the intermediate tube and positioned betweenthe guide tube bushing and the lower bushing of said intermediate tube,said first spacer engaging said guide tube bushing and said lowerbushing when said intermediate tube is in a raised position so as tolimit the upward travel of said intermediate tube, and a second spacerdisposed between the intermediate tube and the inner tube and positionedbetween the upper bushing of said intermediate tube and the bushing ofinner tube, said spacer engaging said upper bushing of said intermediateguide tube and said bushing of said inner tube when said inner tube isin a raised position so as to limit the upward travel of said innertube.
 21. A vertically adjustable column for supporting a seat of achair, the column comprising:a guide tube mounted to a support stand,said guide tube comprising a first retaining member; an intermediatetelescoping tube slidably positioned within the guide tube, saidintermediate telescoping tube comprising an exterior shoulder engagingsaid first retaining member when in a raised position so as to limit theupward travel of the intermediate tube and an interior shoulderpositioned below said exterior shoulder; an inner telescoping tubeslidably positioned within the intermediate tube and comprising an upperportion thereof connected to a seat support member and a secondretaining member engaging said interior shoulder of said intermediatetube when in a raised position so as to limit the upward travel of theinner telescopic tube.
 22. The column of claim 21 further comprising agas spring comprising a cylinder mounted within said inner telescopictube and a piston rod extending from said cylinder in an axial directionand comprising an end connected to said guide tube.
 23. The column ofclaim 21 wherein said exterior and interior shoulders are atsubstantially the middle of the length of the intermediate tube.
 24. Thecolumn of claim 21 wherein said first retaining member comprises acollar mounted to a top portion of said guide tube.
 25. The column ofclaim 21 wherein said second retaining member comprises a collar mountedto a bottom portion of said inner telescopic tube.
 26. The column ofclaim 21 wherein said first retaining member comprises a collar mountedto a top portion of said guide tube and said second retaining membercomprises a collar mounted to a bottom portion of said inner telescopictube.